Vitamin B12

Vitamin B12 is a water-soluble vitamin that is naturally present in some foods, added to others, and available as a dietary supplement and a prescription medication. Vitamin B12 exists in several forms and contains the mineral cobalt, so compounds with vitamin B12 activity are collectively called "cobalamins".

Too Much Vitamin B12

Taking vitamin B12 is generally considered safe. However high-dose supplements or prescription-only injections of B12 might cause diarrhea, itching, blood clots and allergic reactions in some people. If you have a hereditary eye disease known as Leber’s disease, vitamin B12 could seriously harm the optic nerve, which might lead to blindness. People who are allergic to the element cobalt should also avoid taking vitamin B12 because it could cause an allergic reaction.

Vitamin B12 has the potential to interact with certain medications. In addition, several types of medications might adversely affect vitamin B12 levels.